1. Field of the Invention
The invention relates to a process and a field coil for the measurement of the hysteresis curves of magnetic materials. The invention is especially suited for measurements on bar- or strip-shaped specimens of soft magnetic materials, but is also applicable to permanent magnetic materials.
2. Prior Art
It is known that the hysteresis curves of such materials can be determined on test specimens in a elongated field coil. In this case the flux density, B, is measured with a coil surrounding the specimen by integration of the voltage generated by the flux change, dB/dt. The inner field intensity, H, can be calculated from the external field, H', in which the specimen is located, if the demagnetizing coefficient D.sub.H, i.e., the difference between the external field, H', and the inner field H', is known. The external field, H', is determined approximately by the dimensions and the turn density of the field coil, n/1, and by the current I flowing through this coil, and is H'=n.I/1 in long coils. In this known measurement process, errors occur when the test specimen and the field coil are not infinitely long, when the flux density in the specimen is high and if the length/diameter ratio of the specimen is small.
A better-known measurement of the field intensity, H, in the specimen is carried out by means of magnetic voltmeters or potential measuring coils, which are located in the inside of the field coil next to the specimen. These potential measuring coils must, however, have a relatively small cross section, because the field coil and the flux density measuring coil should enclose the test specimen as closely as possible, and therefore the potential coil can furnish only a weak test signal.
It has also been proposed that the demagnetizing coefficient in such a measuring arrangement should be measured with a potential coil, which penetrates the winding of the field coil at two points (Kohlrausch, Prakt. Physik, 1943, Vol. 2, p. 100, FIG. 64). However, it is impossible to measure the inner field intensity solely from the determination of the demagnetizing coefficient since the coefficient is not constant, and is dependent on the flux density, B. Moreover, this known measuring arrangement has the disadvantage that the winding must be interrupted twice to allow the two ends of the potential coil to pass through the winding, and accordingly a considerable error appears at these points in the field intensity of the coil.